1. Field of the Invention
The present invention relates to an image signal reproducing apparatus for generating an image signal by decoding a compressed and encoded image signal.
2. Related Background Art
An orthogonal transformation encoding system has been known as a technique for compressing and encoding an image signal with a high efficiency. In this technique, an image signal is divided into blocks each having a predetermined number of pixels, and they are orthogonal-transformed by such as a discrete cosine transformation (DCT) and the transformed coefficients are quantized and entropy-encoded.
FIG. 1 shows a block diagram of major elements of a digital VTR as an example of an image recording and reproducing apparatus which uses such block encoding and decoding. The flow of the image signal in FIG. 1 will now be explained. Digitized image data is inputted from an input terminal and is groups into blocks of m pixels.times.n pixels by a block forming circuit 2.
The image data blocked by the m.times.n pixel unit (block forming circuit 2) is orthogonal-transformed by a DCT (discrete cosine transformation) circuit 3 so that it is transformed from space area data to frequency area data. The data transformed to the frequency area data is quantized by a quantization circuit 4, and variable-length-encoded by a variable length encoding circuit 5 to attain a desired data transfer rate. Parity data is added to the variable length encoded data by an error correction encoding circuit 6 to error-correction-encode the data, which is formatted to conform to a recording medium by a recording and reproducing apparatus 7 and recorded on a magnetic tape.
In reproduction of the data, the data reproduced from the magnetic tape by the recording and reproducing apparatus 7 is formatted to conform to the signal processing in a succeeding stage, error correction is performed by using the parity data added during the recording by an error correction circuit (ECC) 8, and the data is decoded by a variable length decoding circuit 9. The decoded data is dequantized by an INVERSE-quantization circuit 10 and inversely discrete-cosine-transformed by an inverse DCT circuit 11, and then is converted from frequency area data to space area data and written into a frame memory 12. The image data written in the frame memory 12 is read in synchronism with the raster scan of a monitor, and error-uncorrectable data (hereinafter referred to as or "undesired error data") is interpolated by an interpolation circuit 13 and is then outputted from an output terminal 14 for display on the monitor. As used herein and in the claims, an "undesired error" or "undesired error data" refers to those errors that arise unintentionally during a process such as transmission or reproduction due to noise, random effects or the like. These terms do not include decoding errors that arise from data being encoded using a lossy coding method.
In such an encoding system, since the encoding is done block by block, when an uncorrectable error occurs during the error correction in decoding the encoded data, a decode error occurs block by block. Further, since the variable length code is frequently used, the decoding error propagates over a plurality of blocks, which significantly deteriorates the image quality.
As means for interpolation-correcting the block including the error data (hereinafter referred to as an "error block"), an inter-frame interpolation system in which the error frame is replaced by an image of a preceding frame as shown in FIG. 2 has been known. In FIG. 2, when a block X of a frame #N is undecodable, the data of the block X is replaced and interpolated by decoded data of a block A at the same position on the field in the preceding frame #(N-1).
As an interpolation system which is effective when the error block is dynamic, an intra-field linear interpolation system which interpolates an error block by using pixels in the same field as shown in FIG. 3 has been known. In FIG. 3, upper case letters A, B and X denote encoded blocks and lower case letters a, b and x with suffixes 1, 2, . . . denote lines of reproduced image signals included in the respective encoded blocks A, B and X. The lines with an odd number suffix (shown by a chain line) are in an odd number field, and the lines with an even number line (shown by a broken line) are in an even number field. For the purpose of explanation, it is assumed that the encoded block comprises 8.times.8 pixels (m=n=8) in the frame.
It is assumed that the block X is an undecodable error block and block A and B which are located above and below the block X on the display screen are decodable. Lines x1-x8 of the error block X are replaced by the inter-field linear interpolated values by the bottom lines a7 and a8 of the respective fields of the upper block A and top lines b1 and b2 of the respective fields of the lower block B.
For example, in the odd number field, EQU x1=(4a7+b1)/5 EQU x3=(3a7+2b1)/5 EQU x5=(2a7+3b1)/5 EQU x7=(a7+4b1)/5
and in the even number field, EQU x2=(4a8+b2)/5 EQU x4=(3a8+2b2)/5 EQU x6=(2a8+3b2)/5 EQU x8=(a8+4b2)/5
Thus, in the prior art image signal reproducing apparatus, a state of movement of an error block, that is, a correlation between an image screen (frame) including the error block and the immediately following image, screen is determined, and the two interpolation methods are selectively used.
However, in the digital VTR described above, the movement information for the error block is binary, that is, "move" or "non-move". For the non-move, sharp interpolation may be made, but for the move, the outline of the image for inter-field linear interpolation processing cannot be reproduced, and non-sharp image appears so that visually high-grade image is not reproduced.
Further, in detecting the movement of the error block, the movement between frames is detected based on the data of the frame including the error block and the data which is one frame before. Thus, when the move is detected, even if a correlation between the data of the error block and the data of the image following to the frame including the error block is high and better image will be produced by the interpolation using the data of the following image, the inter-field interpolation is forcibly used, and high quality image may not be reproduced.
Further, when the inter-field-intra-field interpolation is selected, a resolution may be reserved by the inter-frame interpolation but a border image between the error block and the surrounding pixel blocks is discontinuous due to a poor precision of the determination of the movement of the error block, and the reproduced image is unnatural and visually high quality image is not reproduced.